Optical harmonic generation has been known since the experiments of Franken et al. in 1961. Nonlinear optical frequency converters are particularly useful for laser systems which are capable of efficiently producing high-intensity optical fields at a long infrared wavelength, but not at shorter visible or ultraviolet wavelengths. The highest harmonic generation conversion efficiencies to date have been observed using Type II quadrature frequency conversion. This scheme uses two consecutive Type II frequency doubling crystals oriented so that the planes containing their optic axes and the beam propagation vectors are orthogonal. Since, under Type II phase-matching conditions, the input field is polarized at 45.degree. with respect to this plane for the first crystal, the fundamental field which remains unconverted after passing through the crystal is correctly polarized for efficient conversion in the second crystal. Furthermore, the second harmonic field output from the first crystal is polarized incorrectly for optimum phase-matching in the second crystal (otherwise, back-conversion would occur); consequently, it will pass through the second crystal without loss (except for absorption and surface reflections).
Type I quadrature frequency conversion has not been demonstrated to date, because the polarizations of the fundamental input beams must be parallel. Consequently, either the fundamental fields which remain unconverted after passing through the first crystal are incorrectly polarized for efficient conversion in the second crystal (if it has been rotated by 90.degree. relative to the first), or the harmonic field produced in the first crystal will be back-converted in the second crystal (if it has not been rotated by 90.degree. relative to the first).
The overall efficiency of existing high average power visible laser systems is severely limited by the conversion efficiency of the harmonic generator. In the case of all-solid-state lasers, a high efficiency frequency converter will dramatically reduce the number and the cost of the needed diode pump lasers, as well as significantly reduce the operating costs of the overall laser system. Conventional moderate-efficiency frequency converters generally use a single crystal of potassium dideuterium phosphate (or KD*P) and Type II phase-matching. However, many modern nonlinear optical materials provide the most highly efficient conversion of infrared wavelengths to the visible (potassium dihydrogen phosphate, or KDP; lithium triborate, or LiB.sub.3 O.sub.5 ; 5% magnesium-oxide-doped lithium niobate, or 5% MgO:LiNbO.sub.3 ; and potassium niobate, or KNbO.sub.3) and visible wavelengths to the ultraviolet (barium metaborate, or BaB.sub.2 O.sub.4 ; and KD*P) under Type I phase-matching conditions. These materials have sufficiently low threshold powers that highly efficient frequency converters could be produced for a wide variety of industrial, government, and medical applications.